if you want to use ranged weapons, i'd slap a few more low walls for cover. other than that? a battle with lots of pole-arms (halberds, glaives, whatever) would be really fun. minifigs would have a hard time swinging their large weapons in the dense forest, and provides a tactical obstacle all on it's own

I think there's room for plenty of improvement. Personally this looks really slapped together. I don't get the point of the snow. It makes no sense along witht he palm trees and water. The little outpost is a neat idea but it just doesn't fit in this environment. When making terrain you should focus on specific goals, such as creating a choke point, or a fallback position, or a strategic advantage point, or an obstacle for both players for example. Sense it looks like you are trying to build a type of forest environment, you should definitely check out some classic castle creations to get some good ideas on landscaping. I'll post examples. Here comes a lesson.

Look at this picture. There's really nothing outstanding about it, it's a pretty easy build, but look at the thought process here. We have a high tower to bring something different but appropriate to the landscape. Something that stands out and is obvious that players will fight over for control. For cover there are two obvious points. One is the ruined wall and the other is the plant life. We have two points of very dense foliage which would provide ample cover for an attacker trying to get to the wall -- the group of 4 bushes, and the big tree.
http://www.brickshelf.com/gallery/castlebuilder100/Castle/CastleRuins/ancient_ruins_001.jpg

Now here is an easy fix to your plant problem and that damn snow pile. To make it look like a dense forest you can cheat by making fewer trees but just make them bigger, like seen here. Also, you could convert your snow pile into a secret cave of somesort. Who knows, maybe you could build a monster or band of theives to hide in it. In this pic you can see the obvious use of terrain.

Now in this pic it's a bit more parts and skills intensive but just try to look at the basic concepts. This is an example of a choke point. We have two starting positions opposite each other and in the middle is an impassable area except for a bridge -- the checkpoint, where players are forced to move their forces in narrow line. While this creation has its focus on the bridge, you can add this checkpoint concept off to the side of your landscape and have some kind of reward for players that risk going through it, such as it being a shortcut to something or there's a mound of weapons or treasure at the end of it.
http://www.brickshelf.com/gallery/builderslounge/Welcome/DARKspawn/03ichidou.jpg